The General rule of Discharge by Frustration
The law of frustration is governed within the Law Reform (Frustrated Contracts) Act 1943.
If a party to a contract was prevented from keeping their promise because of an unforeseeable, intervening event, then they are not liable for breach of contract. (Taylor v Caldwell)
Due to these unforeseeable events, contracts will often now contain a force majerure clause. Without this clause, the contract will have to rely on the law of frustration to avoid being in breach of contract.