A non-modern one. A Pseudohistorical with a classic monster. This owes a bit to FASA's adventure The Iytean Menace, especially the alternate story hook created by this Actual Play and borrowed by a GM in my playtest group.
1884. A cargo hauler approaches the London docks with a cargo retrieved from a wreck in the Mediterranean. And clawed hands grab the dangling guyrope and something hauls itself up, to bear down on one of the crew...
The TARDIS arrives near the docks, where the ship was hauled in empty with some cargo missing as well as the crew, and traces of blood. The owner of the vanished cargo, shipping magnate Layton Mercier, is furious, claiming that the cargo contained archaeological finds from Atlantis. And 1884's a little early to discover Atlantis, isn't it?
Mercier isn't the sort to answer questions no matter what the psychic paper says the crew's credentials are, as he intends to repair some damaged "Atlantean" weapons and sell them to the Crown, possibly through the Torchwood Institute, who have also shown an interest in the broken diving bell the empty ship was supposed to bring in.
And the creatures searching for the "Atlantean" artefacts, Sea Devils, will stop at nothing to regain them. With it, they can combat humanity's incursions into their habitat. Initially wearing the murdered ship crew's oilskins and sou'westers to disguise themselves, they will take more suitable heavy clothing from victims on land to venture further afield.
Things To Do: Investigate Mercier's estate as well as the deaths around the docks, stop the Sea Devils killing anyone who gets too near as well as attacking Mercier and his men - who have a Silurian subsonic blaster or two ready to use as well as more conventional weapons.
Antagonists: Sea Devils, Mercier's men, possibly Torchwood agents.
Action Scenes: A chase through the docks, a submersible attack, dodging blaster fire.
Problems: Neither the Sea Devils or Mercier feel much like listening to reason. And the authorities are taking an interest as well. And the "Atlantean" weapons might not be entirely stable.
Things To Prepare For: Mercier wants to impress someone with his cleverness - possibly someone in authority, but maybe he wants to make his fortune to catch someone's eye. And his estate is big enough that there are a few ways in and out. And if you really want to go big, the Sea Devils could have brought a working submersible, ready to haul itself on land and attack the city.
Continuing the Adventure: Where did Mercier really get his finds from? Is there a would-be Captain Nemo out there, really searching for Atlantis, and what else might he bring to the surface?
And a handout!