We present the design of a mid-IR (5 - 28 micrometers ) integral field spectrometer for NGST. Details of the opto-mechanical design are given with particular attention being paid to those aspects influenced by the cryogenic, low background space environment in which the instrument needs to work. The instrument consists of three subsections: fore-optics and image slicing integral field units (IFU's), a 5 - 10 micrometers spectrograph and 10 - 28 micrometers spectrograph. Two co-aligned fields of view are separated into two wavelength channels (5 - 10 and 10 - 28 micrometers ) by a dichroic mirror in the fore- optics which also re-image the telescope focal plane onto the slicing mirrors of two IFU's.