Terahertz (THz)-band communication is envisioned as a key wireless technology to satisfy the need for much higher wireless data rates. To date, major progress in electronic, photonic and plasmonic technologies is finally closing the so-called THz gap. However, the exceedingly large available bandwidth at THz frequencies comes at the cost of a very high propagation loss. Combined with the power limitations of THz transceivers, this results in very short communication distances. Moreover, the absorption by water vapor molecules further splits the THz band in multiple transmission windows, which shrink as the transmission distance increases. To overcome these limitations, the concept of Ultra-Massive Multi-Carrier Multiple Input Multiple Output (UMMC MIMO) communication, which relies on the use of ultra-dense frequency-tunable plasmonic nano-antenna arrays, has been recently proposed. In this paper, the end-to-end performance of a UMMC MIMO link is analytically and numerically investigated. More specifically, an optimization framework is developed to determine the information capacity of UMMC MIMO communication by taking into account both the capabilities of THz plasmonic nanoantenna arrays and the peculiarities of the THz channel. In relation to the arrays, the frequency tunability of each individual element in the transmitter’s and receiver’s plasmonic arrays is taken into account. In terms of the channel, the impact of the spreading loss and the molecular absorption loss is considered. Extensive numerical results are provided to illustrate the performance of the proposed communication scheme.