From Law wiki, the wiki for law research

A form of the general Tort of Interference with goods, in which the defendant wrongfully takes possession of another person's goods. Conversion overlaps with Theft, of course, but conversion can be undertaken without the element of dishonesty that characterises theft. Conversion often arises in the context of (innocently) receiving goods that have been obtained dishonestly. For example, if A uses an assumed identity to convince B to accept a worthless cheque in payment for goods, then the contract between A and B might be void for mistake. If so, then A does not obtain title to the goods. If A then sells them to C, a bona fide purchaser, C does not obtain title either. As a result, B can sue C in conversion to recover the goods. This is very hard on C, who has usually lost his money and the goods, but the losses have to fall somewhere, and generally A has done a runner.

To simplify the law, the tort of conversion is now defined by statute (TortsInterferenceWithGoodsAct1977) to include what used to be called Detinue.