How to plot current in ngspice?
Oli gave a correct answer but the I(element_name) is an extension added only to the commercial SPICE versions.

In ngspice (which is based on Berkeley Spice 3) you can only plot currents through (independent) voltage sources. These are the only currents that appear in the circuit equations SPICE works from.

In an interactive Spice session or from a special block in the script (see also this question) you can use expressions like (v(1)-v(2))/1k when the current is through a 1kΩ resistor between nodes 1 and 2. For reactive elements (like a 1μF capacitor) something like (v(1) - v(2))/(2*pi*frequency*1u) should  also work.