Plot Tunnel

Most RPGs are free-roaming affairs: Between the scripted cutscenes and predetermined plot points that take place at designated locations, you are free to Take Your Time, get Sidetracked by the Gold Saucer, and generally go Dungeon Crawling anywhere your current abilities allow (assuming it isn't blocked off by a Broken Bridge).

At least, until you enter a Plot Tunnel: A tightly linear, Plot-driven sequence within an RPG where your usual ability to freely explore areas and perform Side Quests is denied you in favor of whatever plot-dictated events shall unfold. There are No Sidepaths, No Exploration, No Freedom within a Plot Tunnel; your only objective is to proceed straight ahead while the plot develops (and possibly twists). You will be able to return to your usual, player-driven exploration routine only after you've exited out the other end.

Can be considered a gentle form of Railroading, in that the player has no choice except to Follow the Plotted Line. When a Plot Tunnel occurs during the final act of a game, it becomes the Point of No Return.

Note that since dungeons come in all shapes, sizes, and lengths, a general guideline for identifying a Plot Tunnel is that it should be at least two dungeons long, with no ability to access the Hub Level or use your Warp Whistle in the meantime.

Examples of Plot Tunnel include:

Baldur's Gate 2 has a fairly long one once you Get on the Boat. There's a town with some Side Quests, a fairly long dungeon that ends with a Final Boss Preview, optionally a Monster Town, a trip to the Underdark which has two more towns, three more dungeons and several more side quests before you finally return to the surface and can access all other places again. Most of the Underdark section can actually be skipped if you know where to go, but the game doesn't tell you this. The whole sequence covers two of the seven chapters of the story.
In Breath of Fire IV, near the end of the first chapter you set foot on the Evil Empire's homeland for the first time to search for your missing princess. Although you do have access to the World Map, you have no Warp Whistle at this point, and the only location you may travel is a nearby town and waterway, which you use to sneak in to the Imperial base in search of the princess. This ends with the party being discovered, captured and deported as the game begins Chapter Two.
Early on in Final Fantasy XII, Vahn sneaks into the Dalmascan palace through the Garamsythe Waterway. This leads to a series of events in which he meets up with sky pirate Balthier, the party is captured and imprisoned in Nalbina Fortress, escapes through the Barheim Passage and must head back to Rabanastre (the Hub Level) from the eastern desert.
Final Fantasy XIII is rather infamous for putting the player through a Plot Tunnel that starts with the Prolonged Prologue (two chapters long) and ends by Opening the Sandbox... eight chapters after that. That's right, you spend ten chapters out of thirteen with No Sidepaths, No Exploration, No Freedom. The base was slightly broken about that, to say the least.
There are a few sequences in the Pokémon Mystery Dungeon series where you must progress through multiple dungeons without access to the usual shops, the Hub Level, your mission list, or being allowed to recruit new team members.
- In the first installment (Red and Blue Rescue Team), at one point your party is Misblamed for the disasters plaguing the world and run out of town on a manhunt, leading to a sequence of five Dungeon Crawls (including two Boss Battles) before the matter is resolved and you are allowed to return home.
- In the second installment (Explorers of Time and Darkness), the Guild makes an expedition to Fogbound Lake (a total of four dungeons and one Boss Battle). Later, an unexpected trip to the future turns the entire plot upside down across a sequence of four dungeons; even after returning to the present time, you must progress through an additional dungeon before you are allowed to return to the Guild and resume your usual routines.
The Horizon and the Collector Ship mission of Mass Effect 2 count, since both will block all exploration and side-quests after recruiting the fourth and the seventh squad member, respectively, until you complete them. In this case, the Plot Tunnel begins in the respective previous mission, though you don't know that on your first play-through.
Suikoden V contains a variant of this. During certain plot events, some allies cannot be sought and recruited.