Someone stole the body
The objection
This is the oldest explanation on record — Matthew reports it circulating already — and its age is a point in its favor, not against it. Bodies are portable. Tombs are not vaults. The disciples had motive: a movement that ends with an executed leader is finished, and a movement with a risen leader is not. Grave robbery was common enough in antiquity that an emperor is thought to have issued an edict against it. And you do not even need the disciples: gardeners, family, or the authorities themselves could have moved the body for entirely ordinary reasons, after which a group of grieving followers found an empty tomb and drew the conclusion they wanted.
The response
Split the theory in two, because the halves fare differently. The deliberate-fraud version has to explain why men who knew it was a hoax accepted beatings, imprisonment, and death for it — and why Paul, who was on the other side and had every reason to expose the trick, joined them instead. People do die for falsehoods; they very rarely die for a falsehood they personally manufactured. The innocent-relocation version is stronger and cannot be dismissed. Its weakness is not the empty tomb but the appearances: a moved body explains an empty tomb and explains nothing else, and it is exactly the correction the authorities could have made simply by producing the body once preaching began in Jerusalem. Where this lands: theft is a live explanation of the tomb and a poor explanation of everything after it.