We introduce a Stochastic Two-Dimensional Fluid Model that consists of two stochastic fluid flows driven by the same underlying Markov chain, where one of the fluids is unconstrained. We develop the theoretical and numerical framework for the transient analysis of the model. We derive the important generator matrix of a particular Laplace-Stieltjes transform of the model, which is the foundation of our analysis. We use it to develop expressions for the Laplace-Stieltjes transforms of various performance measures for the transient analysis of the model and construct powerful algorithms for their numerical evaluation. An example of an application in a queueing environment is given.