Sam Mills’s virtuosic new novel is – when defined in the strictest terms – a romp. By that I mean it’s an adventure story that doesn’t ask you to take it all that seriously. The Count of Monte Cristo is the definitive romp: a tale of repeated imprisonment and escape, of thwarted romance, of daring disguises and, in the end, of triumphant human grit and ingenuity. The Watermark has all that, but with added metatextuality and time travel. If you love Doctor Who, you will love this book. It whirls you off on a similarly breathless Technicolor tumble through different eras and genres. But where the Doctor has the Tardis, the two main characters of The Watermark – journalist Jaime and painter Rachel – have cups of magical tea.

The tea is administered to them by an obviously rompish baddie, Augustus Fate. Fate is a bestselling but extremely bitter author, living in rural Wales, who has realised after seven Booker prize shortlistings that his novels lack convincing characterisation and genuine emotion. His solution is to lure two real people to his remote house, and then, by means of the magical tea, to sedate, brainwash and insert them into his stalled work in progress, Thomas Turridge. This is a poor Victorian pastiche set in 1860s Oxford. Jaime will become the titular Thomas; Rachel will keep her real name. Neither of them will remember anything of who they are or what’s happened to them before. Instead, Fate can put them through terrible trials, and note down what they say and do as they react. A prize-winning literary masterpiece is almost bound to ensue, if he can only keep the police at bay and his two bedbound prisoners unconscious.

However, because this is a romp, Jaime and Rachel are gradually able to wake up within Fate’s world. They begin to hear the narrator saying things like, “And so Thomas kissed Rachel and they burnt with a fiery, illicit passion.” Here is where the novel is really clever – because it forces us to read extremely attentively. Each anachronism, everything that fits our world but not Victorian times, is a sign of Jaime’s genuine self struggling to break through. Eventually, with the crashing arrival of a helicopter in the middle of a church service, we get the full world-splitting effect. Fate is thwarted, temporarily at least, and Jaime and Rachel are able to flee into another book – this time set in a poorly imagined 2010s Manchester. Poorly imagined, because its author is their friend and adviser from within the first story, Mr James Gwent, apparently a man of the 1860s but actually an earlier abductee of Fate’s.

This section is one of the novel’s many highlights. Mills has a great deal of fun with the limitations of Gwent’s imagination. When Jaime and Rachel try an excursion to St Petersburg, their flight becomes increasingly sketchy. “I point at the window. The scenery outside has been leached of colour. Our seats are no more than pencil strokes; the view from the window is reduced to a draft.” Three further jumps occur, with the lovers book-surfing to Soviet Carpathia in 1928, to a robot-dominated London of 2047, and one more destination – I won’t spoil things by mentioning the finale.

Throughout, Mills brings to bear the skills learned in crafting her earlier genre-hopping and head-spinning novels, particularly The Quiddity of Will Self, and her more recent memoir about caring for a parent with schizophrenia, The Fragments of My Father. Metafiction, under which category The Watermark certainly falls, is often seen as a heartless literary game. But there’s nothing cold about Mills’s writing. Exactly what August Fate lacks, and needs to steal from others, she gives away with great generosity. The watermark that runs through the whole novel is the troubled love between Jaime and Rachel. Whatever reality they inhabit, he will always appear carrying a bird and she will always reluctantly but passionately fall for his foolishness. They will bicker. They will split up. They will disagree about the fundamentals of life: Jaime will be optimistic and rational; Rachel will be near-suicidal and yet believe in higher forces. But they will always stumble back together. They fight their way out of each new prison of clunky worldbuilding, and reestablish their true, flawed relationship.

Various phrases echo through their story, reminding the lovers of the reality they started from. The one that recurs most piercingly, rather like Alice in Wonderland’s “You’re nothing but a pack of cards!”, is the call to disillusion, the call to life. “This is not real. This is a backstory being wrapped around my soul. You’re in a book, you’re in a book.” But what a book to be in.