I've mentioned the Ship of Theseus before. I first heard of it as 'the Grandfather's Axe Problem' and in the UK now is often known as 'Trigger's Broom'.
The idea is simple. You replace all the timbers and rigging of a ship. When is it not the same ship? When you replace the handle and head of an axe, does it stop being the same axe? When Trigger has replaced the handle and bristles of his broom several times, is it the same broom?
A similar thing can happen to bands. Napalm Death, Christian Death and Dr Feelgood, for example, somewhat famously continued even though there were no original members left. A party is like that sometimes. Members come and go, either because of player drop-outs or PC death (even without going into TPKs). After a while a party may have little or no resemblance to the original membership.
So, in this instance, is it the same party? And, yes or no, is it possible for an ever-changing constellation of characters to have a 'quest'?
As a thought-experiment going back to the Lord of the Rings...
Session 1 - The Shire (Hobbiton to Crickhollow):
Three players - Alan (Frodo), Barbara (Sam) and Charles (Pippin) decide they want to run 'Ring-Quest': the DM tells Frodo's player that his uncle gave him a magic ring before disappearing into the Wild, and Frodo is going to find out what happened to him. If the Hobbits had (for example) lost Pippin to a Tiger Beetle attack in Woody End (I once rolled up encounters for the Fellowship as per the Mentzer wilderness rules and this happened on the first night, as described in the reply to this post in 3d6, Traps & Thieves), they could then have been joined by Gildor (Charles thought he'd play an Elf next session).
(This part has been edited to make it more true as well as functional.)
Session 2 - The Old Forest (Crickhollow to Tom's House):
Frodo, Sam and Gildor are joined by Deirdre playing Merry, but Merry is killed in the Old Forest. The rest of the party makes it safely to Tom's House, and Deirdre rolls up a Magic User, Goldberry, who will join the next leg of the journey.
Session 3 - The Barrow Downs (Tom's House to Bree):
The party departs from Tom's House. They get involved in a dungeon-delve, but Barbara's character Sam is killed by the Barrow-wights. Barbara rolls up a Ranger for her next character (maybe in Basic D&D terms this is a Cleric, but Barbara says she really thinks he should have a sword. The DM agrees, as long as it's broken and can't be fixed until Rivendell) as the party continues on to Bree.
Session 4 - Midgewater (Bree to Weathertop):
By the time they are leaving Bree the party is Frodo (still Alan's first PC and still the Ringbearer), Strider (Barbara's second PC), Gildor (Charles's second PC) and Goldberry (Deirdre's second PC). The quest continues on to Weathertop, where there is a big fight with the Nazgul and Frodo is injured (Strider the Cleric can only use a torch as a weapon).
Session 5 - the Trollshaws (Weathertop to Rivendell):
Another player, Erik, joins them for this session, with Glorfindel as his PC (an Elf scouting from Rivendell), and the quest continues to head in that direction. Again there is a confrontation with the Nazgul but the party makes it to Rivendell. At Rivendell after 5 sessions, Frodo's player Alan decides to jack it in. Frodo, it is decided, will stay in Rivendell healing from his wound. The DM tells Alan that Frodo's uncle is there, and the two Hobbits, in game terms at least, retire to study Elvish poetry. By now there are no Hobbits in the party. The PCs have completely changed from the group that left Hobbiton and entered the Old Forest. Strider (replaced Sam as Barbara's PC), Gildor (replaced Pippin as Charles's PC) and Goldberry (replaced Merry as Deirdre's PC), with Glorfindel (Erik's PC) however, still constitute a party. Alan agrees Frodo will hand over the Ring to Goldberry.
Session 6 - Moria (Rivendell to Lorien):
This session starts with a Ring-Quest party forming with Goldberry, Gildor, Strider, Glorfindel, Gloin (played by Faith), Gimli (played by Gary) and Boromir (played by Helen). Oh and an NPC Gandalf. Faith and Gary aren't really sure they're going to be able to stick around, but they've come along for a session anyway. Now apart from the fact that only Strider and Boromir have names that don't begin with 'G', that seems like a viable party, though maybe Gloin at 235 (40 years older than Thorin at the time of The Hobbit) is a bit old to be going adventuring. The party leaves Rivendell and travels towards Moria. Fights with the Watcher in the Water, a bit of riddling to find the secret door, and some fights in the tunnels with Orcs lead to a confrontation on the bridge where Glorfindel (Erik's PC) falls fighting the Balrog. The two Dwarven players decide they won't be coming back and drop out of the campaign. They decide that their characters are going to continue to fight in Moria against the Orc Hordes, trying to re-take it for the House of Durin; the rest of the party 'knows' they're still alive (in game terms) somewhere in Khazad Dum. Erik really wants to keep playing even though Glorfindel is dead, and likes Elves, so when the party rests up in Lorien, Haldir joins the party. However, Gildor's player Charles (who played Pippin originally) also drops out of the campaign and Gildor 'stays in Lorien', mourning the death of his friend Glorfindel.
Session 7 - Anduin I (Lorien to Amon Hen):
By this point eight players have been involved. Two of the three players from the first session have dropped out (Alan and Charles AKA Frodo and Pippin/Gildor). But the quest continues. Deirdre is keeping the quest alive with her character Goldberry acting as the Ringbearer. The party heads south - with Goldberry, Strider, Haldir and Boromir (with NPC Gandalf of course). The party reaches Amon Hen, where they are ambushed by Orcs. Goldberry is killed. The rest of the party send her body over the Falls of Rauros in an Elven boat. Before she dies she gives the Ring to Strider. He, Boromir and Haldir vow to continue.
Session 8 - Anduin II (Amon Hen to Cair Andros):
About to depart from Amon Hen, the PCs meet a Rohirric commander, Eomer (the PC of Iain, a new player), who is in the area on patrol. He joins the party. They journey downriver towards a Gondorian garrison near Cair Andros: on the way, they raid an Orcish stronghold in a cave-system near a Gondorian fort. Here, Strider is killed fighting a giant horned troll, but he impresses on Eomer the importance of the quest, and he takes the Ring. There's a whole bunch of caves here, enough for several sessions adventuring from this Gondorian keep on the borderlands if the party wishes, but having scouted the caves the party continues south.
Session 9 - Ithilien (Cair Andros to Osgiliath):
At Cair Andros, the party is joined by a new PC. Charles has decided that he wants to give the campaign another go so he rolls up a new character - a Thief, who he decides is a Gondorian soldier called Beregond. The DM decides that Beregond's commanding officer has sent him with the party as a guide to the next garrison, at Osgiliath. The PCs make their way to the ruined city and fight Orcs and Nazgul on the way.
Session 10 - Morgul Vale (Osgiliath to Minas Morgul):
At Osgiliath, two more PCs join the quest. These are Ioreth, a Cleric run by Deirdre, who re-joins the campaign, and Eowyn, a fighter, run by Jeanette, a new player. Ioreth is a healer attached to the Osgiliath garrison. The DM rules that a force from Rohan has arrived to bolster the Gondorian forces, which has included Eowyn disguised as a man. They join Eomer, Boromir, Haldir and Beregond in an journey to the fortress of Minas Morgul, where a Gondorian army is encamped in Morgul Vale. On the way they are ambushed by Southrons coming to break the siege, and nearly caught by the Witch-King's magic - but Eowyn eventually kills the Witch-King with a little help from Haldir. However, both Haldir and Eomer are killed; with his dying breath Eomer gives the Ring to Boromir.
Session 11 - Cirith Ungol (environs of Minas Morgul):
Not yet tiring of the shenanigans in Morgul Vale, the party decides at the next session to go on a little side-quest up into the hills. Erik, now that Haldir is dead, rolls up another Elf (he really likes Elves) called Imrahil. The DM says that there really aren't any Elves in the Gondorian army, but Eriks says "can he be a sort of Half-Elf... using the rules for an Elf?" Oh, all right says the DM. However Charles (the only original player, who had actually dropped out of the campaign for a bit) can't make this session. The DM rules that Beregond is busy with his military duties - in game terms, he is spending the day reporting to the commanders of the Gondorian army about the Mordorian forces to the north and the state of the defences. Nevertheless, Boromir, Eowyn, Iorweth and Imrahil go off into the hills above the Morgul Vale. There they meet a giant spider and some Orcs, and have some fun dungeon-bashing, before coming back to camp.
Session 12 - Gorgoroth (Minas Morgul to Orodruin):
Charles unfortunately can't make this session either (so Beregond is still busy doing stuff for the army, probably scouting). Boromir the Ringbearer, Eowyn, Ioreth and Imrahil head off across Gorgoroth to take the Ring to Mt Doom. They have some trouble with Orc patrols and have to evade Nazgul but eventually they get there. Boromir makes a speech remembering all the brave souls who have helped them and casts the Ring into the fire. Then the PCs are racing a volcano to get the hell out of there! Frankly at this point even if the PCs do snuff it they've already saved the world so they can congratulate themselves on a heroic sacrifice well-done. Haldir and Eowyn both fail their Save v Dragon Breath to avoid the fumes. Boromir and Ioreth decide they'll carry their unconscious brethren so the DM makes them take another Save - they both make it and the DM rules that they narrowly get their companions out of the danger zone. There's little point in role-playing the journey back so the DM concludes that at last, weary and dehydrated, they stagger into a Gondorian patrol who are amazed that they had not perished. Returning to a jubilant camp, they are told that a heroes' welcome awaits them in Minas Tirith.
12 sessions. 10 players. 17 PCs (and Gandalf as an NPC). In that time the party fulfilled the point of 'Ring-Quest' and had a bunch of exciting adventures. None of the original PCs, or even players, finish the campaign. In a few places, the DM had interpret the actions of players that had an effect on characters (such as Beregond being absent). It's not quite as polished as Lord of the Rings but it's not bad I don't think. It's closer to 'heroic fantasy' than most D&D games I've played.
I've been trying to find a format for a travelling 'quest'-style game where the party is always changing. Even typing this out has made me realise something. Back in the old days when most of the 'scenarios' I read came from White Dwarf, there was a clichéd opening that was 'travelling all day you at last come to a village at sunset. As you make your way to the inn...'. Sometimes it was just 'Finding yourself in an inn...'. I used to wonder why. I've realised that 'the inn' in which the party finds itself is (spiritually) 'The Prancing Pony'. The things happening in the scenario weren't the 'quest': they were incidents along the way. They were the Barrow-wights' tombs. They were the diversions to Cirith Ungol. The 'quest' was the reason the party was always travelling. The way to get an episodic 'quest' with a changing party is to end each session (as the Rift City campaign ends) back at safety, at the next Inn, at the Keep, at the army camp, at the Homely House. There the PCs can rest and personnel can be swapped.
Every session needs to be like this - and the PCs either return to where they started to try again tomorrow, or go to the next point down the line. Bag End - Crickhollow - Tom's House - Bree - Weathertop Camp - Rivendell - Lorien - Amon Hen - Cair Andros - Osgiliath - Minas Morgul - Minas Morgul again: then Orodruin for a big climax. That's a fair campaign and allows plenty of opportunity for replacement PCs or new players to integrate themselves into the action.
The rules seem simple. If a PC is left behind, they're left behind, and they need a reason to do so, which means the DM needs to tell a story why. If the player comes back later, they need a new PC. If the quest has some McGuffin, the Guardian of the McGuffin must be appointed and successors designated as needed. Beyond that it just (just!) requires some flexibility by the DM. Parties can stay in one place (more adventures around Minas Morgul) or move on (in the example above the PCs barely glimpsed the Caves of Chaos before heading to Osgiliath). But whichever way, there must always be the possibility of integrating new PCs at the beginning of every session.
'Points of Light' + Nodes = 'Nodes of Light'. Cool. That might work.