Cassandra is a scientific data viewer based on VTK. Cassandra provides dynamic interaction with the VTK pipeline and enables the user to load plug-ins dynamically in order to perform specific tasks in data manipulation and visualisation. Cassandra is open and modular, and can easily be extended to new filters through a set of dynamically loadable plug-ins. A plug-in template is provided to help users to develop and share their own plug-ins. This approach allows users to perform complex filtering without any specific knowledge of VTK or C++.