Henry Wilt is an unremarkable and uninspired lecturer at a rundown community college in the UK. His marriage to his wife, Eva, is falling apart as she constantly belittles and dominates him. At a university party, Henry gets stuck in an inflatable doll and is unable to extricate himself in time, leaving him humiliated in front of his colleagues.
After the party, Henry decides to get rid of the doll and dumps it in a hole at a building site. However, he is seen by two young boys who mistake him for a murderer. One of them, named Sid, takes the doll and hides it in his treehouse.
The next morning, Henry wakes up to find that Eva has disappeared. Inspector Flint, a determined police officer, begins investigating the case and quickly zeros in on Henry as the prime suspect. Henry's odd behavior and the fact that he was seen getting rid of the doll make him a person of interest to the police.
Meanwhile, Henry teams up with his colleague, Mrs. Prout, to investigate the mysterious disappearance of Eva. He poses as a private investigator and they begin following various leads that take them to a mental institution and a seedy bar.
As the investigation unfolds, Henry becomes increasingly worried that he will be arrested for his wife's disappearance. He even goes so far as to consult a lawyer, who advises him to turn himself in to the police. But Henry's troubles are far from over as the real kidnappers are revealed, leading to a dramatic showdown and a surprising twist.
In the end, Henry is exonerated and his wife's kidnappers are brought to justice. The film concludes with Henry and Eva renewing their wedding vows, suggesting that their troubled relationship has been restored.