Door.

It's this bit with The Mushrooms, isn't it?

"The Head came through 

The Table and said, 

`I'm from The Future.  I've come 

to Save Your Life.  

We  found A Stasis Leak 

on Floor-Sixteen.'" 

You see, I don't think it was 

An Hallucination --

When You saw Your Head 

coming through The Table, 

I Think You were seeing 

You, as You are now, 

arriving back in The Past.

Oh, A Magic Door!  

Well, why didn't you say?

9 Int. Level 16 corridor. Present.

He beckons to RIMMER and CAT.

LISTER: 

It's safe.  Come through.

Derry Girls | Erin's Prom Date

10 Int. Shower. Past.

He steps back through the leak.  RIMMER and CAT follow him. 

 They all emerge in the same shower cubicle.

CAT: (to RIMMER) 

What IS it?

RIMMER: 

It's A Rent in the Space-Time continuum.

CAT: (to LISTER) 

What IS it?

LISTER: 

The stasis room freezes Time, 

you know, makes Time stand still --

  So whenever you have A Leak, it must 

preserve whatever it's leaked into, 

and it's leaked into this room.

CAT: (to RIMMER) 

What IS it?

RIMMER: 

It's A Singularity, a point in The Universe 

where the normal Laws of 

Space and Time don't apply.

CAT: (to LISTER) 

What IS it?

LISTER: (wearily)

It's A Hole back 

into The Past.

CAT: 

Oh, A Magic Door!  

Well, why didn't you say?

Acathla, The Demon came forth to swallow 

The World. He was killed by a
virtuous knight who pierced

 the demon's heart, before 

he could draw a breath 

to perform The Act -- 

Acathla turned to stone, 

as demons sometimes do, and 

was buried, where neither Man 

nor Demon would 

ever want to look --

.......unless of course they're putting-up 

low-rent housing. Boys...