High resolution transit spectroscopy has proven to be a reliable technique for the characterization of the chemical composition of exoplanet atmospheres. Taking advantage of the broad spectral coverage of the CARMENES spectrograph, we initiated a survey aimed at characterizing a broad range of planetary systems. Here, we report our observations of three transits of tplanet with CARMENES in search of het absorption. On one of the nights, the He~{sc i} region was heavily contaminated by OH$^-$ telluric emission and, thus, it was not useful for our purposes. The remaining two nights had a very different signal-to-noise ratio (S/N) due to weather. They both indicate the presence of het absorption in the transmission spectrum of tplanet, although a statistically valid detection can only be claimed for the night with higher S/N. For that night, we retrieved a 1.5$pm$0.3% absorption depth, translating into a $R_p(lambda)/R_p = 1.15pm 0.14$ at this wavelength. Spectro-photometric light curves for this same night also indicate the presence of extra absorption during the planetary transit with a consistent absorption depth. The het absorption is modeled in detail using a radiative transfer code, and the results of our modeling efforts are compared to the observations. We find that the mass-loss rate, mlr, is confined to a range of 3,$times,10^{10}$,gs for $T$ = 6000,K to 10,$times,10^{10}$,gs for $T$ = 9000,K. We discuss the physical mechanisms and implications of the He~{sc i} detection in tplanet and put it in context as compared to similar detections and non-detections in other Neptune-size planets. We also present improved stellar and planetary parameter determinations based on our visible and near-infrared observations.
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