Considering the Internet-of-Things, the blockchain technology has recently become a major focus of research. One of its major drawbacks is given by the poor performance of blockchain systems subject to heavy load. In particular, systems which use Proof-of-Work as leader election strategy are not suitable for an active participation of IoT devices. We propose a new system architecture which uses the Plasma framework to integrate blockchain technology and fog computing and evaluate the performance of its prototype. The Plasma framework has the advantage to provide a scalable hierarchical design based on sidechains and an off-chain scaling strategy which is agnostic with regard to the architecture of the employed root chain.