We present Advanced Camera for Surveys observations of MACS J1149.5+2223, an X-ray luminous galaxy cluster at z = 0.544 discovered by the Massive Cluster Survey. The data reveal at least seven multiply imaged galaxies, three of which we have confirmed spectroscopically. One of these is a spectacular face-on spiral galaxy at z = 1.491, the four images of which are gravitationally magnified by 8 lsim μ lsim 23. We identify this as an L sstarf (MB sime -20.7), disk-dominated (B/T lsim 0.5) galaxy, forming stars at ~6 M sun yr-1. We use a robust sample of multiply imaged galaxies to constrain a parameterized model of the cluster mass distribution. In addition to the main cluster dark matter halo and the bright cluster galaxies, our best model includes three galaxy-group-sized halos. The relative probability of this model is P(N halo = 4)/P(N halo = 1012 where N halo is the number of cluster/group-scale halos. In terms of sheer number of merging cluster/group-scale components, this is the most complex strong-lensing cluster core studied to date. The total cluster mass and fraction of that mass associated with substructures within R